Unfounded fiscal fears may be preventing Florida farmers from implementing fertilization programs that could have a positive effect on the environment, researchers say.

Controlled-release fertilizers that can benefit the environment while maintaining yields have been around for years. But growers often are scared off by the cost, which can be one and one-half to two times the cost of soluble fertilizer. However, that’s down from five times the cost of soluble products just a few years ago.

While the per-unit cost might be greater than with soluble fertilizers, the long-term costs are similar, growers and researchers say.

The issue is not whether controlled-release fertilizers work, says Eric Simonne, former Extension specialist, vegetable crop production at the University of Florida’s Institute of Food and Agricultural Sciences. The challenge is selecting the right one and incorporating it into a controlled release-based fertility program.

“Having as little soluble fertilizer in the field as possible is desirable because, if it rains, the soluble fertilizer is gone,” he says. “If the prills [fertilizer granules] are coated with polymers or sulfur, then it’s protected.”

Simonne, who specializes in vegetable production, says the challenge in selecting a product for use with vegetables is finding something that has a fast release, since vegetables typically grow in 30 to 90 days.

“You don’t want an 18-month product,” he says.

Growers must know the product’s length of release and how it responds to the temperatures under which it will be applied, since the release rate depends on temperature.

Blends are best

The best way to use controlled-release fertilizers is in a blend with regular soluble fertilizers, says Kelly Morgan, associate professor in the soil and water science department at the University of Florida’s Southwest Research and Education Center in Immokalee. The center is conducting research to determine the optimal ratio of the two products.

About half of the state’s growers use some amount of controlled-release fertilizer, he says.

The highest ratio is about 25 percent controlled-release product with the rest soluble. The average is about 15 percent. In most cases, it’s all applied at one time during planting.

Controlled-release fertilizer prolongs the amount of time fertilizer is available to the crop and helps maintain water quality since, if soluble fertilizer washes away with the rain, it degrades the quality of the water, he says.

Duda’s program

Oviedo, Fla.-based A. Duda & Sons Inc. uses some type of controlled-release fertilizer on its 8,000 acres of citrus—mostly juice oranges and some grapefruit and tangerines—says Joby Sherrod, research and development manager.

The company didn’t jump on a controlled-release program overnight, though.

Duda has been using controlled-release materials grove-wide for three years, Sherrod says, but the firm experimented with the process for at least five years before implementing it on a large scale.

“We found blending to be better than using straight controlled-release material alone,” he says.

The company worked with the manufacturer, which worked with the distributor to come up with the desired mix of controlled-release and standard soluble fertilizers.

“It gets very detailed, but we’ve honed in on a recipe that works for us,” Sherrod says.

Duda evaluated a variety of materials and release curves and conducted in-house trials, modifying as it went, before deciding on a preference. The firm continues to tweak the program.

There is a difference

Controlled-release products work in slightly different ways, Simonne says.

With a sulfur-coated product, there is a shell and, through diffusion of water in the shell, pressure increases, the shell explodes, and there’s a “catastrophic release” at the prill level.

“What makes the change in release occur is the different thicknesses of the coating,” he says.

Polymer-coated materials have a kind of plastic mesh spread over the prills, and there’s a “leak-type release,” he says.

Since the polymer expands under warm conditions and retracts when it’s cool, polymer-coated materials may be more sensitive to temperature changes, he says.

You can’t say one material works and another doesn’t, Simonne says. You must find out which one works for you, how it releases in comparison with the window of need of the plant and determine whether it’s economically doable.

“With fertilizer, it’s always an issue of rate [pounds of material per acre], source [the chemical form of the element you are trying to deliver to the plant, such as ammonium nitrate or urea], placement and timing,” he says. “Controlled-release fertilizer is only as good as it helps you answer those four questions.”

Research has not been completed on all of the options available, he says, so growers must do their own experimenting.

You can’t simply go by what a manufacturer claims, he says, because often, manufacturers “just want to sell you a product.”

Choose carefully

Tomatoes and peppers are primary winter vegetables in Florida, Morgan says. He recommends using a fertilizer that is not urea based, since studies have shown that, under certain conditions, the urea releases very rapidly and turns into ammonia, damaging the plants.

“Ammonia toxicity will kill [tomatoes] very quickly,” he says, and peppers also are somewhat sensitive to ammonia.

Other types of controlled-release fertilizers did little damage to the plants in tests, “but the release rates weren’t quite right,” he says. “We don’t have everything exactly right yet, but we’re working on it.”

On the citrus side, most growers use CitriBLEN for young citrus trees that need nutrients on a regular basis, Morgan says. In the past, growers had to apply nutrients every two- to four weeks because the sandy soil does not hold nutrients well.

“CitriBLEN has helped the growth of the trees and helped reduce labor costs tremendously,” he says.

Once trees are 3 to 5 years old, growers typically go back to standard soluble fertilizer, he says.

Fertilizer efficiency

Duda now uses less total nitrogen with controlled-release fertilizer and achieves the same fruit output for the same cost as with soluble fertilizer, because the company was able to achieve better fertilizer efficiency, Sherrod says.

“We’re pretty pleased with it so far,” he says.

But the program wasn’t perfect.

Duda used to make four applications during the year, but reduced that to three with a controlled-release blend.

The company tried using just one annual application but found the fruit color was not up to its standards at certain times of the year.

“We didn’t feel we were getting the release at the appropriate times,” Sherrod says. “The release curve did not appear to fit the growth cycles of the citrus.”

It did not affect yields, however, and the problem was solved after the company increased the number of applications.

Duda also is experimenting with its celery program, but the challenges with vegetables are different from those of citrus, he says. And it’s too early how well a controlled-release program will work with veggies.

“Vegetables require a large release of nutrients early on and quickly,” Sherrod says. “If the controlled-release material isn’t designed for that, you’re not going to get the release that the plant requires in that timely manner.”

Fertilizer makers still have some work to do when it comes to vegetables, he says.

“The manufacturers have to figure out the right method for release and dial in the release curve for those controlled-release materials to match the crop,” Sherrod says.  

Contact Citrus + Vegetable at vlboyd@att.net or (209) 571-0414.